The field of art to which this invention pertains is the hydrocracking of a hydrocarbonaceous feedstock. Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline by hydrocracking a hydrocarbon feedstock derived from crude oil, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 700xc2x0 F., usually at least about 50 percent by weight boiling above 700xc2x0 F. A typical vacuum gas oil normally has a boiling point range between about 600xc2x0 F. and about 1050xc2x0 F.
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs and higher liquid product yields. It is generally known that enhanced product selectivity can be achieved at lower conversion per pass (60% to 90% conversion of fresh feed) through the catalytic hydrocracking zone. However, it was previously believed that any advantage of operating at below about 60% conversion per pass was negligible or would only see diminishing returns. Low conversion per pass is generally more expensive, however, the present invention greatly improves the economic benefits of a low conversion per pass process and demonstrates the unexpected advantages.
U.S. Pat. No. 5,720,872 discloses a process for hydroprocessing liquid feedstocks in two or more hydroprocessing stages which are in separate reaction vessels and wherein each reaction stage contains a bed of hydroprocessing catalyst. The liquid product from the first reaction stage is sent to a low pressure stripping stage and stripped of hydrogen sulfide, ammonia and other dissolved gases. The stripped product stream is then sent to the next downstream reaction stage, the product from which is also stripped of dissolved gases and sent to the next downstream reaction stage until the last reaction stage, the liquid product of which is stripped of dissolved gases and collected or passed on for further processing. The flow of treat gas is in a direction opposite the direction in which the reaction stages are staged for the flow of liquid. Each stripping stage is a separate stage, but all stages are contained in the same stripper vessel.
International Publication No. WO 97/38066 (PCT/U.S. Ser. No. 97/04270) discloses a process for reverse staging in hydroprocessing reactor systems.
U.S. Pat. No. 3,328,290 (Hengstebech) discloses a two-stage process for the hydrocracking of hydrocarbons in which the feed is pretreated in the first stage.
U.S. Pat. No. 5,980,729 (Kalnes et al) discloses a hydrocracking process utilizing reverse staging in hydroprocessing reactor systems and a hot, high-pressure stripping zone.
The present invention is a catalytic hydrocracking process which provides higher liquid product yields, specifically higher yields of turbine fuel and diesel oil. The process of the present invention provides the yield advantages associated with a low conversion per pass operation without compromising unit economics. Other benefits of a low conversion per pass operation include the minimization or elimination of the need for inter-bed hydrogen quench and the minimization of the fresh feed pre-heat since the higher flow rate of recycle liquid will provide additional process heat to initiate the catalytic reaction and an additional heat sink to absorb the heat of reaction. An overall reduction in fuel gas and hydrogen consumption, and light ends production may also be obtained. Finally, the low conversion per pass operation requires less catalyst volume.
In accordance with the present invention, the undesirable production of polynuclear aromatic compounds is controlled by removing a small dragstream of high pressure product stripper bottoms to reject polynuclear aromatic compounds and recovering valuable diesel boiling range hydrocarbons and unconverted feedstock by routing the dragstream to a hot flash separator and subsequently to a divided wall fractionation zone to produce a concentrated stream of polynuclear aromatic compounds while recovering the valuable hydrocarbon compounds.
In accordance with one embodiment the present invention relates to a process for hydrocracking a hydrocarbonaceous feedstock which process comprises: (a) passing a hydrocarbonaceous feedstock, a hydrocracking zone effluent and hydrogen to a denitrification and desulfurization reaction zone containing a catalyst and recovering a denitrification and desulfurization reaction zone effluent therefrom; (b) passing the denitrification and desulfurization reaction zone effluent directly to a hot, high pressure stripper utilizing a hot hydrogen-rich stripping gas to produce a first vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia, and a first liquid stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock; (c) passing at least a portion of the first liquid stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and hydrogen to a hydrocracking zone containing a hydrocracking catalyst and recovering a hydrocracking zone effluent therefrom; (d) passing the hydrocracking zone effluent to the denitrification and desulfurization reaction zone; (e) passing the first vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia to an aromatic saturation zone containing hydrogenation catalyst to produce a second liquid stream comprising hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock and having a reduced concentration of aromatic compounds; (f) passing at least a portion of the second liquid stream produced in step (e) to a first zone of a divided wall fractionation zone to recover at least a portion of the hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock; and (g) passing hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and polynuclear aromatic compounds from the bottom of the hot, high pressure stripper to a second zone of the divided wall fractionation zone to produce a third liquid stream comprising polynuclear aromatic compounds.
Other embodiments of the present invention encompass further details such as types and descriptions of feedstocks, hydrocracking catalysts and preferred operating conditions including temperatures and pressures, all of which are hereinafter disclosed in the following discussion of each of these facets of the invention.